What home radon monitors don't monitor.

What home radon monitors don't monitor

The Public Health Service recommends that homeowners monitor for radon-222 and take corrective action where levels of this natural, radioactive pollutant prove high (SN: 9/24/88, p.206). The concern is not the radon itself but the hazard it signals: decay productS, called "daughters," that account for roughly 55 percent of human radiation exposure and up to 20,000 lung cancers annually in the United States. However radon monitors don't assay alla the features that determine the threat these daughters pose. Reports in the June HEALTH PHYSICS highlight two such features: the presence of radon-220 and the fraction of daughters that do not cling to dust.

The radons descend from different families. Radon-220 is a fifth-generation daughter of thrium-232, and radon-222 is a sixth-generation descendant of uranium-238. As gases, each of these is the only member of its family's decay chain that can percolate from its source in rocks or soil through cracks into a building.

Radon-220's fleeting nature explains why most current radon monitoring ignores it, says Dowell E. Martz, a physicist with the U.S. Department of Energy in Grand Junction, Colo. Radon-222's 3.8-day half-life means that a week's collection in charcoal canisters -- the most common home monitors -- yields enough for a reasonable gauge of room levels. But with a half-life of only 55 seconds, most of the radon-220 collected will decay before a laboratory analyzes the canisters, Martz explains. Even the more sensitive "alpha-track" monitors can easily miss this radon, because it will not last long enough to disperse throughout a room. Unless one looks for its longer-lived (and better dispersed) daughters -- something not usually done -- documenting radon-220's brief appearance requires blanketing a room with monitors.

Martz and his co-workers designed a portable spectrometer to record week-long, round-the-clock emissions of alpha radiation in eight Colorado buildings. Using the known characteristic alpha energy of each radon daughter, they established that roughly one-quarter of the alpha radiation in the buildings' air had been emitted by daughters of radon-220.

Accounting for these emissions should not increase the estimates of lung dose to occupants of these buildings by more than about 8 percent, observers Naomi Harley, a radiation oncologist at New York (City) University Medical Center. That's because she and others have shown that radon-220's daughters deliver only one-third the dose to the lung that radon-222's do, per unit of exposure. But she adds that "this doesn't necessarily mean [radon-220 is] a nonproblem." White Colorado's geology does not include much thorium (radon-220's progenitor), large thorium deposits in other areas, including Canada, might make radon-220 a serious indoor hazard, she says.

Another HEALTH PHYSICS report from researchers at Georg August University in Gottingen, West Germany, indicates that the tiny fraction of radon daughters that don't attach to airborne dust in relatively clean rooms is three times higher than the value ordinarily used to estimate radiation doses to room occupantS. The scientists found rates reaching the standard value only in rooms with large sources of aerosols such as heaters, burning cigarettes, lit candles, cooking, or excessive outdoor ventilation.

Quantifying this "unattached" fraction of daughters is important because virtually all of them, when inhaled, will deposit their radiation in the upper bronchial tree, where most lung cancers begin. In contrast, fewer than 5 percent of the heavier, slower-moving dust-bound daughters tend to irradiate the lungs, Harley notes. The new data suggest that human-dose estimates for any given radon level in a relatively aerosol-free environment should be increased, though "not dramatically -- certainly less than 25 percent," she says.